ooops. Missed the '80 computers' part... :-)
looking at what passes for documentation for at.exe I don't see anything specifying the user. That's no good. But I do see that you can specify the computername, which makes me wonder... how does AT verify that I have permission to run this program on that other computer? My only guess is that it uses WinAuthentication using my credentials. Which makes me think that unless you specify the process to be interactive, it will run under the username of the person who called AT. Its a big jump, but you might want to test it out... set a job for five minutes from now and then log out and log in as someone else and wait to see what happens.
Just a thought.